The speed is much faster with SitetoSite when I turned the WireGuard VPN off. However with this, I am not able to use my phone with the Wireguard app.

I there any way of adding a smartphone to S2S?

Can I use both, the WireGuard Server and SitetoSite?

Also, the USB tethering only works for a few minutes and then it is automatically disconnected in the Travel router settings.